1. Field of the Invention
The present invention relates to light emitting diodes and manufacturing methods thereof. More specifically, the present invention relates to a homoepitaxial ZnSe-based light emitting diode (hereinafter, referred to as an LED) which has a ZnSe single crystal substrate and a light emitting structure including a mixed crystal compound semiconductor made of ZnSe or mainly of ZnSe, as well as a manufacturing method thereof.
2. Description of the Background Art
ZnSe-based LEDs emitting green light are well known. The LED has a light emitting structure formed on a ZnSe substrate and an electrode formed thereon.
The LED is formed by depositing various semiconductor layers on the ZnSe substrate such as by epitaxial growth, forming an electrode thereon, cutting them into a chip of a prescribed size, and then fixing it to a lead frame.
However, defects such as dislocations and cracks, which are caused during cleavage, densely exist near the cleavage plane of the LED after being cut. If a current is passed through such a region having a large number of defects, not only is the light emission efficiency decreased but the density of defects increase as a consequence of the current injection resulting in short life of the element.
It can be considered that the defects near the cleavage plane can be addressed by etching and removing a portion near the cleavage plane. In this case, however, it is necessary to cope with a side etching phenomenon which is caused by a difference in the etching rates of the semiconductor layers.
The present invention was made to solve the above described problems. An object of the present invention is to provide a higher luminance value and a longer life for an LED without using complex processes.
A light emitting diode according to the present invention includes a ZnSe substrate and a light emitting structure formed on the substrate. It is noted herein that the light emitting structure refers to a stacked structure of semiconductor layers including a light emitting layer. The light emitting diode having the above described structure includes a current restriction region for suppressing the increase in the density of defects by restricting the path of the injected current for light emission.
By thus providing the injected current restriction region, it is possible to restrict or prevent a current flow into the region and to suppress the increase in the defect density in the region. Especially by selecting a region having a large number of defects as the injected current restriction region, it is possible to realize a longer life and a higher luminance value for the LED.
Preferably, the above described light emitting diode is a homoepitaxial ZnSe-based light emitting diode. Preferably, the ZnSe substrate is a conductive ZnSe single crystal substrate, and the light emitting structure includes a mixed crystal compound semiconductor made of ZnSe or mainly of ZnSe.
The present invention is particularly effective for the homoepitaxial ZnSe-based light emitting diode as described above.
Preferably, the light emitting diode includes a cleavage plane for cutting an element to be separated. At this time, the injected current restriction region preferably includes a region near the cleavage plane.
As described above, a large number of defects may exist near the cleavage plane of a light emitting diode. Therefore, by suppressing a current flow into such a region having a large number of defects, it is possible to realize a longer life and a higher luminance value for the LED.
The light emitting diode has an ohmic electrode (also called the xe2x80x9csecond electrodexe2x80x9d herein) on the light emitting structure. Preferably, th e ohmic electrode or second electrode is positioned to be apart from the above described cleavage plane.
By providing the ohmic electrode to be apart from the cleavage plane, it is possible to suppress a current flow into the vicinity of the cleavage plane.
It is therefore possible to realize a longer life and a higher luminance value for the LED. Since the position of an end of the electrode only needs to be adjusted, the LED can be given a longer life and a higher luminance value without complex processes.
Specifically, the above described second or ohmic electrode is formed to be isolated from the cleavage plane by at least 5 xcexcm, and the diode has a further first electrode for wire bonding.
Since a particularly large number of defects may exist in a region within 5 xcexcm from the cleavage plane, the longer life of the LED can be effectively provided by not forming the ohmic electrode for diffusing an injected current at least in this designated region.
Preferably, the above described second or ohmic electrode is a light transmitting electrode and includes at least one material selected from the group consisting of Au, Pd, Ni and ITO (Indium Tin Oxide).
The light emitting diode according to the present invention may have, on the light emitting structure, a non-ohmic first electrode made of a material not attaining ohmic contact with the light emitting structure for restricting an injected current, and an ohmic second electrode made of a material attaining ohmic contact with the light emitting structure for diffusing an injected current. It is preferred in this case that the first electrode is formed on the region near the cleavage plane and the second electrode is formed on a region other than the region near the cleavage plane.
Since the first electrode does not attain ohmic contact with the light emitting structure, a current flow from the first electrode into the light emitting structure can be suppressed. In other words, a current flow into a region under the first electrode can be suppressed. It is therefore possible to suppress an increase in defects in the region under the first electrode. By providing the first electrode on the region near the cleavage plane, for example, it is possible to prevent a current flow into the region near the cleavage plane and to suppress an increase in defects in the region. On the other hand, a current can be applied from the second electrode into the light emitting structure, causing the LED to emit light.
The light emitting diode may have a semiconductor layer selectively provided inside the light emitting structure and not attaining ohmic contact with the first electrode, and a trench portion provided at an upper surface peripheral portion of the light emitting structure and extending to the cleavage plane and the semiconductor layer. In this case, the first electrode is formed on the trench portion to be in contact with the semiconductor layer.
In this case as well, it is possible to suppress a current flow into the region under the first electrode and to suppress an increase in defects in the region under the first electrode. Preferably, the trench is formed by etching. More specifically, the trench is preferably formed by such etching that emphasizes irregularity on the trench surface, for example, ion milling and reactive ion etching.
Preferably, the above described first electrode is formed at least on a region within 5 xcexcm from the cleavage plane and includes at least one material selected from the group consisting of Ti, Al, ZnS, Al2O3, SiO2 and SiN.
By forming the first electrode on the region as described above, it is possible to prevent a current flow into the region within 5 xcexcm from the cleavage plane.
Furthermore, a current suppression layer for suppressing a current flow may be provided inside the light emitting structure near the cleavage plane.
By thus providing the current suppression layer inside the light emitting structure, it is possible to suppress a current flow into the region under the current suppression layer. It is therefore possible to suppress an increase in defects in the region.
The light emitting diode may include a first electrode for restricting an injected current, provided on such a surface portion of the light emitting structure that is located on the above described current suppression layer and made of a material not attaining ohmic contact with the current suppression layer, and a second electrode for diffusing an injected current, provided on such a surface portion of the light emitting structure that is located on a region where the current suppression layer is not formed and made of a material attaining ohmic contact with the light emitting structure.
By thus providing the first electrode using the material not attaining ohmic contact with the current suppression layer, it is possible to suppress a current flow into the region under the current suppression layer even if a certain conductive layer exists between the first electrode and the current suppression layer. It is therefore possible to suppress the increase in the density of defects in the region under the current suppression layer.
In one aspect, a light emitting diode manufacturing method according to the present invention includes the steps of forming a light emitting structure on a ZnSe substrate; forming, on a first surface region of the light emitting structure, a first electrode made of a material not attaining ohmic contact with the light emitting structure for restricting an injected current; and forming, on a second surface region of the light emitting structure, a second electrode made of a material attaining ohmic contact with the light emitting structure for diffusing an injected current.
By thus forming the first electrode made of the material not attaining ohmic contact with the light emitting structure on the first surface region, it is possible to suppress a current flow into a region immediately under the first surface region. It is therefore possible to suppress the increase in the density of defects in the region. Since the second electrode made of the material attaining ohmic contact with the light emitting structure is formed on the second surface region different from the first surface region, a current can be supplied from the second electrode into the light emitting structure, causing the LED to emit light.
In another aspect, a light emitting diode manufacturing method according to the present invention includes the steps of: forming a light emitting structure on a ZnSe substrate by sequentially stacking a first semiconductor layer including a light emitting layer, selectively a second semiconductor layer on the first semiconductor layer, and a third semiconductor layer from the top of the semiconductor to the top of the second semiconductor layer; forming, on such a surface portion of the light emitting structure that is located on the second semiconductor layer, a first electrode made of a material not attaining ohmic contact with the second semiconductor layer for restricting an injected current; and forming, on such a surface portion of the light emitting structure that is located on a region where the second semiconductor layer is not formed, a second electrode made of a material attaining ohmic contact with the light emitting structure for diffusing an injected current. It is noted that the above described first to third semiconductor layers are a stacked structure of a plurality of semiconductor layers.
By thus selectively forming the second semiconductor layer on the first semiconductor layer and forming the first electrode thereon, it is possible to suppress a current flow into a region immediately under the semiconductor layer. It is therefore possible to suppress an increase in defects in the region under the semiconductor layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.